Derivative Activities - Effects of Non-Hedge Derivatives on Consolidated Statement of Operations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	$ (24,109)	$ 137,760	$ (170,959)	$ 40,776
Gain (Loss) Recognized in Income (Ineffective Portion)	0		0	(2,891)
Derivative Fair Value Income (Loss)	(24,109)	137,760	(170,959)	37,885
Swaps
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	(38,521)	65,003	(82,593)	21,927
Gain (Loss) Recognized in Income (Ineffective Portion)				(1,995)
Derivative Fair Value Income (Loss)	(38,521)	65,003	(82,593)	19,932
Re-purchased swaps
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)		(1,663)		(478)
Derivative Fair Value Income (Loss)		(1,663)		(478)
Collars
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	1,032	74,420	(38,116)	19,417
Gain (Loss) Recognized in Income (Ineffective Portion)				(896)
Derivative Fair Value Income (Loss)	1,032	74,420	(38,116)	18,521
Basis swaps
Derivative Financial Instruments [Line Items]
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	13,380		(50,250)	(90)
Derivative Fair Value Income (Loss)	$ 13,380		$ (50,250)	$ (90)